This invention pertains to articulated adjustable arms and, more particularly, to such arms which can be freely movable and maintained in desired positions. Adjustable articulated arms have many uses. A very desirable use is for a lamp to place the necessary working light where it is needed. The two most prominent methods to effect such support are (1) directly friction loading to the joints so that they grip to support the load and (2) counterbalancing the load by use of springs.
The least responsive method is the first since the articulated arms have locking or gripping joints that hold the load of the lamp by force or structural strength. To adjust the lamps that use this method, a user must release the grip or lock in the joint, support the hold while the adjustment is made and then relock the grip to hold the arm in the new position. A variation of this method is shown in U.S. Pat. No. 3,820,752 in which three radii are gripped and held in position by a cable that wraps at least once around a drum fastened to each joint and grips the drum as tension is applied to the end of the cable by means of a spring. The tension applied to the cable need only be sufficient to cause the cable to bite the drum. This patent likens the principle of the support of the arm to the method by which a ship can be held at dock by a rope wrapped once or twice around a bollard and then held by a workman. In this patent, the drum acts as the bollard and the spring as the workman. To adjust this support the tension of the cable must be released in order to free the drums from the grip of the cable, allowing the arm to be repositioned. The cable tension must then be reapplied to grip the drums and hold the arm in its new position. Such a scheme is impractical when frequent adjustments of the lamp position are required since each adjustment necessitates the routine of unlocking, supporting the lamp while repositioning and relocking after the new position is reached.
Lamp arms that are properly counterbalanced can be adjusted without releasing any mechanisms to swing the joint for movement. Friction can be applied in the joints to control the ease of motion but not so much friction as to lock or force the joints to grip in position. Examples of such spring counterbalanced lamp arms can be found in U.S. Pat. Nos. 2,395,178 and 3,374,347 wherein the base and middle joints are springloaded and friction is applied to aid in the counterbalancing. A problem with lamps of this type is that when the position of the lamp arm is adjusted the angle of the lamp head or bulb housing changes and must be adjusted to compensate for the change. A very good variation of this principle is shown in the well known Luxo-type arm which employs four large external springs, three structural members in the base section and two structural members in the top portion of the arm. The support members are fastened at the joints in such a way that when the arm is moved, the lamp head remains at a constant angle. The large external springs and friction in the joints counterbalance the load. The Luxo-type has now been modified to a type called the "K" arm that requires only two base structural members and two members in the top portion of the arm and has internalized springs in the support members. In both versions, the principle that allows the lamp head angle to remain constant is the same. These two types of lamp arms are the most widely used types available; however, the Luxo-type lamp has several drawbacks in that it requires many different parts. In addition, the joint configuration places restrictions on the movement of the arm. There is also objection to its exposed parts which collect dirt.